The ratios of the Si IV 1393.75 Å and 1402.77 Å resonance lines are shown in Fig. REF . There, the diamonds indicate the measured ratios at different positions along the slit. They were color-coded in order to distinguish between the ratios measured in (blue) and away (red) from the reconnection region. For optically thin plasma, this ratio should be equal to 2 [1]}, [2]}, which we indicated using grey dashed line in Fig. REF . Even though all of the observed ratios are below this value, the ratios measured farther from the reconnection region (red diamonds) are consistently closer to 2 than those measured in the reconnection region (blue diamonds). The computed ratio range for the red points,
away of the reconnection site is between \(\approx \) 1.72 and 1.95. The maximum values are around 1.9 for points 79 and 80 which infers to us the ability to derive the electron density.
For the blue points in the mini-flare area at the reconnection time, the low ratio value of Si IV lines (1.48 to 1.62) with a high uncertainty does not allow us to compute the electron density
[2]}.
More-less the Si IV line profile shapes are similar to the Mg II line shapes (Fig. REF ) with similar extended blue wings which means that such large profiles has several components like the Mg II lines containing certainly the emission of the flare plus the emission of a cloud with high velocities as we have concluded by analysing the Mg II profiles with the cloud model. It is nearly impossible to distinguish the two components in each Si IV profile and therefore to compute the optical thickness of the cloud and the flare region.
The behaviour of Si IV line in flares is exactly in a similar way as predicted in the theoretical models [2]}, that there is a stratification in the heights at which the various lines (Si IV, C II, Mg II) form, which varies with time in the flare. Initially the core of the Si IV 1393.75 line forms highest in altitude. Toward the end of the heating phase the compression of the chromosphere results as the lines formation in a very narrow region, which persists into the cooling phase. We believe to this stratification of heights of formation of Si IV, and C II lines during the flare. As it is
suggested
by [2]} the Si IV intensity ratio represents the ratio of the source function of the lines, the magnitude of the lines depending on the temperature of the layer where they are formed. The thermalization of both Si IV lines would occur higher in the atmosphere where the temperature is larger. Tests using the RADYN code should be used to understand such low ratio values in term of opacity effect.
One minute later after the flare this effect is negligible.
